- Practice · 5 marks
Clutch versus brake and self-energising
Short answer
A clutch transmits torque between rotating shafts; a brake dissipates kinetic energy to stop a member, with one side fixed. Self-energising: friction helps apply the shoe, increasing torque. Self-locking: friction moment ≥ normal moment, causing lock-up without force. Leading shoe is self-energising; trailing shoe is not, ensuring stability.
Differentiate between a clutch and a brake. Explain the terms self-energising and self-locking in brakes, and the difference between the leading and trailing shoes of an internal expanding brake.
Clutch vs brake
| Point | Clutch | Brake |
|---|---|---|
| Function | Connects or disconnects two rotating shafts to transmit power | Slows or stops a moving member by absorbing its energy |
| Members | Both members are free to rotate | One member is fixed (frame) |
| Energy | Energy is transmitted, small slip heat during engagement | Kinetic or potential energy is converted into heat |
| Design basis | Torque capacity, wear | Energy absorption, temperature rise, heat dissipation |
| Example | Plate, cone and centrifugal clutch | Drum, disc and band brake |
Self-energising
A brake is self-energising when the friction force on the shoe produces a moment about the pivot that helps the actuating force press the shoe against the drum. This increases the normal force and therefore the friction and the braking torque for the same actuating force. Self-energising occurs when the friction moment acts in the same sense as the actuating moment.
Self-locking
If the friction moment exceeds the moment of the normal force, , the shoe pulls itself into the drum even with zero (or negative) actuating force and the brake locks the drum. It is dangerous: it gives jerky braking or cannot be released. It occurs for a large coefficient of friction or when the pivot is badly placed.
Leading and trailing shoes
In an internal expanding brake:
- The leading (primary) shoe has its pivot at the trailing end, so the drum drags the shoe into contact. It is self-energising: more braking torque for the same force.
- The trailing (secondary) shoe has the pivot at the leading end, so the drum rotation tends to drag the shoe away from the drum. It is non-self-energising: friction reduces the pressure.
Hence the leading shoe does more work and wears faster; a brake with a leading and a trailing shoe is stable, which avoids the self-locking risk of two leading shoes.
✓ Answer checked by Claude Opus 5.5 (high effort)
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